1. Field
The present disclosure relates to a method for manufacturing a thin film solar cell and a module structure of a thin film solar cell, and more particularly, to a method for manufacturing a thin film solar cell and a module structure of a thin film solar cell, in which a substrate-incident laser is applied in a scribing process of a thin film solar cell.
2. Description About National Research and Development Support
This study was supported by the Source Technology Development for the Convergence of Renewable Energy program of Ministry of Trade, Industry & Energy, Republic of Korea (Project No. 1415132283) under the superintendence of Korea Institute of Science and Technology.
3. Description of the Related Art
A Se-based or S-based thin film solar cell made of Cu(In1-x, Gax)(Se,S)2 (CIGS) or Cu2ZnSn(Se,S)4 (CZTS) is prepared by depositing molybdenum (Mo) on a glass substrate, a metal substrate or a polymer substrate as a rear electrode, then forming a CIGS thin film (or, a CZTS thin film) as a p-type light absorption layer, and successively depositing a CdS thin film (or, ZnS, In2S3) as a buffer layer, an intrinsic ZnO (or, ZnMgO) as a high-resistive window layer and a transparent conductive oxide (TCO) electrode as an n-type window layer. In comparison to an existing silicon solar cell, the Se-based or S-based thin film solar cell allows reduction of production costs due to its thin film manufacturing and high photovoltaic conversion efficiency (PCE) of 20% or above, and thus this is evaluated as a potential candidate in the next-generation solar cell market.
As an advantage in manufacturing a module of a thin film solar cell, the module may allow for monolithic integration with a structure depicted in FIG. 1. When manufacturing the thin film solar module, each unit cell is isolated by performing a first patterning process P1 to a Mo rear electrode, performing a second patterning process P2 after deposition of CIGS/CdS/i-ZnO, and performing a third patterning process P3 after deposition of TCO, and adjacent cells are electrically connected in series through a TCO/Mo connection of the area of the second patterning process P2. FIG. 1 shows a part of the entire module in which three cells are connected in series for example, and the monolithically integrated module is configured by iteratively forming the structure of FIG. 1 with a serial connection.
FIG. 2 is a diagram showing a preparation process for forming an existing module structure of CIGS thin film solar cell. First, after a molybdenum (Mo) rear electrode is deposited to a substrate, the rear electrode is electrically isolated through the first patterning process P1 using a laser scriber. After a light absorption layer, a CdS buffer layer and an i-ZnO layer are deposited thereon, they are removed through the second patterning process P2 using a mechanical or laser scriber exposing the surface of the rear electrode. After a transparent electrode layer is deposited, it is electrically isolated through the third patterning process P3 using a mechanical or laser scriber. At this time, in the area of the second patterning process P2, the TCO layer serving as an upper electrode and Mo serving as a rear electrode come into contact with each other, thereby connecting the adjacent cells in series.
The mechanical scriber applied to the second patterning process P2 and the third patterning process P3 needs somewhat low equipment costs in comparison to the laser scriber. However, except for this, this mechanical scriber has a disadvantage of causing a problem in productivity due to an increase of defective products since scribing width and quality is degraded by an abrasion of a scribing tip during a module manufacturing process, and thus the tip should be exchanged frequently, thereby increasing the manufacturing costs. In addition, when patterning is performed by means of mechanical scribing, it is very difficult to decrease a line width to a certain level or below, and thus a dead area where a photocurrent cannot be collected increases, thereby decreasing a power amount generated by the module.
To solve this problem, it is needed to develop a technique capable applying a laser scriber to the second patterning process P2 and the third patterning process P3. Compared with the first patterning process P1, in case of the second patterning process P2 and the third patterning process P3, laser is incident from a transparent window layer. At this time, in order to decrease a loss of the Mo rear electrode, the heat transfer to the Mo rear electrode caused by laser heating should be restrained. For this, an expensive pulse laser source having a pulse width of below several ten picoseconds (ps) should be applied.
In addition, reaction products on the Mo surface, which may be generated when a CIGS thin film is removed by laser heating, increase a resistance of the TCO/Mo contact in the area of the second patterning process P2, which in turn increases a series resistance of the module. In addition, debris, ridges, or burs generated during the scribing process may cause shunting among cells or irregularity in following thin film deposition.
The photovoltaic module has extended its market beyond the traditional solar energy generation plants for large-scale electricity production, into a building-integrated photovoltaic module (BIPV), a vehicle-integrated photovoltaic module (VIPV) for a vehicle such as a car or a bus, a device-integrated photovoltaic module (DIPV) for a device with portability. Among them, the building-integrated photovoltaic module (BIPV) may be applied to a roof, a wall and a window of a building, among which the window-type photovoltaic module needs light transmission to a certain level for lighting.
An amorphous thin film Si solar cell removes a light absorption layer by means of laser scribing to ensure transparency, and a crystalline Si wafer solar cell ensures transparency by using a gap to which cells are not applied, thereby meeting their demands. Since persons inside the building are sensitive to the color of the light transmitted through a window in an aesthetic aspect, such a transparent photovoltaic module should be able to provide an option for tuning the wavelength and intensity of the light transmitted through it.